energies

Article

Efficiency Analysis of the Main Components ofa Vertical Closed-Loop System in a BoreholeHeat Exchanger

Cristina Sáez Blázquez *, Arturo Farfán Martín, Ignacio Martín Nieto, Pedro Carrasco García,Luis Santiago Sánchez Pérez and Diego González-Aguilera

Department of Cartographic and Land Engineering, University of Salamanca, Higher Polytechnic School of Avila,Hornos Caleros 50, 05003 Avila, Spain; afarfan@usal.es (A.F.M.); nachomartin@usal.es (I.M.N.);retep81@usal.es (P.C.G.); lssanchez@usal.es (L.S.S.P.); daguilera@usal.es (D.G.-A.)* Correspondence: u107596@usal.es; Tel.: +34-67-553-6991

Academic Editor: Rajandrea SethiReceived: 22 November 2016; Accepted: 3 February 2017; Published: 10 February 2017

Abstract: In vertical closed-loop systems, it is common to use single or double U-tube heat exchangersseparated by longitudinal spacers. In addition, the helical-shaped pipe is another configuration thatrequires lower drilling lengths but it is less used. The aim of the present research is to study theinfluence of these components on the total efficiency of a borehole heat exchanger (BHE). Thus,the differences between using single/double U-tubes (with or without spacers) and helical pipesare analysed in terms of efficiency. Through different laboratory tests, a small vertical closed-loopsystem was simulated in order to analyse all these possible configurations. The grouting materialsand the temperatures of the ground were modified at the same time in these tests. Regarding the heatexchange process between the ground and the heat carrier fluid, it must be highlighted that the bestresults were obtained for the helical-shaped pipe configuration. Some of the improvements offeredby this heat exchanger typology with respect to the vertical configuration is that a lower drillingdepth is required even it requires a larger diameter. This leads to significant economic savings inthe performing drilling process. Finally, it is also worth noting the importance of using spacers invertical U-tubes and that no improvements have been found regarding the use of single or doubleconfiguration of U-tubes. Thanks to the laboratory results derived from this study it is possible toestablish the optimum behaviour pattern for the entire vertical closed-loop systems.

Keywords: vertical closed-loop systems; U-tube heat exchangers; helical-shape pipe; groutingmaterials; heat carrier fluid; borehole heat exchanger (BHE)

1. Introduction

Renewable energies are getting more and more important to address the increasing demand ofenergy. With respect to the geothermal energy, the number of installations has increased over the pastdecade and continues growing [1]. A conventional closed-loop system of very low temperature iscommonly used to heat/cool a certain space or to produce Domestic Hot Water (DHW) [2]. Focusingon the geothermal systems that use closed-loop heat exchangers [3] and in particular on thosevertical borehole heat exchangers; they are usually constituted by vertical pipes, generally madeof polyethylene PE 100 PN16. A heat carrier fluid (usually a mixture of water and glycol) flows insidethese pipes behaving as thermal transmitter between the ground and the rest of components of theinstallation [4–6]. The length of these drillings typically varies from 60 to 300 m (when using verticalpipes), according to the energetic needs to cover in each certain case. For these vertical installations,the most common configurations in relation to the drilling diameter and the design of heat exchangersare presented in Table 1 [7].

Energies 2017, 10, 201; doi:10.3390/en10020201 www.mdpi.com/journal/energies

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Table 1. Usual configurations used in vertical closed-loop systems, X = Pipes with spacers, 7 = Pipeswithout spacers.

Borehole Diameter (mm) Type of Tube Tube Diameter (mm) Spacers

127 Single U 32 or 40 X/7127 Double U 32 7152 Double U 32 X200 Double U 40 X

Another less known design is the helical heat exchanger. This type of pipe with diameters ofaround 400 mm requires much smaller drilling lengths and they are equally made of high qualitypolyethylene. These helical pipes can be installed in boreholes of only 3–5 m although drilling lengthscan vary and be higher depending on the installation in question [8–10].

In both types of configurations (vertical or helical), the space between the hole wall and the heatexchangers is filled by a thermal conductive material to ensure the thermal exchange between groundand fluid inside the pipe. This material, known as geothermal grout, must have a series of mechanicaland thermal characteristics [11,12].

Particular attention is needed for the following factors:

• Compactness, to guarantee the borehole stability and its easy injection to the hole.• Sealing ability, providing a hydraulic barrier that avoids the pollution of aquifers.• Low hydraulic conductivity.• High thermal conductivity for an efficient heat transfer between the pipe and the ground.

As a rule, it is recommended that the grouting material has a higher thermal conductivity valuethan the thermal conductivity of the ground to guarantee an efficient working of the borehole heatexchanger. In any cases, both thermal conductivities should be the highest possible. Some relationsbetween the ground and the grout should be implemented. When the thermal conductivity ofthe ground is <2 W/mK, the thermal conductivity of the grout should be ≥ than the thermalconductivity of the ground and when the thermal conductivity of the ground is ≥2 W/mK,the thermal conductivity of the grout should be ≥2 W/mK [13].

The main objective of this research is to study the efficiency of different designs depending on theheat exchangers and the grouting material used. Through the representation in laboratory of a series ofconfigurations commonly used and other more innovative systems, it was possible to select the designthat provides the most favourable results in the thermal exchange with the ground constituting at thesame time the best solution to achieve the greatest possible effectiveness for this kind of installations.A good decision in the design of the geothermal heat exchangers and the grouting material could meanimportant economic savings since the total drilling length could be reduced [14].

Several studies have been found about the comparison of the different heat exchangers consideredin this manuscript and the main decisive parameters in a borehole heat exchanger. Zarrella et al. [15]concluded that the thermal performance of the helical-shaped pipe was better than the double U-tubeheat exchanger. When the helical heat exchanger was 33% shorter than the double U-tube, each heatexchanger provided the same thermal performance; it denotes that this non-conventional type of heatexchanger could be used to reduce installation costs. Han and Yu [16] emphasized the importance ofoptimizing the thermal conductivity and specific heat capacity of backfill materials. Congedo et al. [17]resolved that the most important parameter for the heat transfer performance of the system is thethermal conductivity of the ground around the heat exchanger. They also stablished that the optimalground type is that with the highest thermal conductivity.

However, most of these studies only offer a theoretical simulation not based on the practicalexperience as this study does [18–20]. On top of that, this study combines different heat exchangerswith different grouting materials and different ground temperatures which allow making a completecomparison and description of a borehole heat exchanger.

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2. Materials and Methods

The experimental methodology consisted of the representation in laboratory of a vertical boreholeheat exchanger with a drilling length of only one meter. From this installation, a set of configurationswere tested for selecting the one that, contributes to increase the total efficiency of the geothermalsystem. The comparison among the different cases of study has been made based on the differenceof temperatures of the heat carrier fluid in the entry and exit of the borehole. When this difference ishigh, the efficiency of the thermal exchange between the ground and fluid is also considerably high.The heat extraction from the ground by the heat carrier fluid is determined by the grouting material(responsible for the heat transmission) and the heat exchanger. Thus, the higher the difference oftemperature between the hot and the cold tube of the heat exchanger, the greater the amount of heatcollected by the circulating fluid. This fact constitutes an indicator of the heat exchanger and groutingmaterial quality [21,22].

Components used to create the borehole heat exchanger and the configurations tested in thelaboratory are presented in Section 2.1.

2.1. Components Used in the Laboratory Tests

A vertical borehole heat exchanger was reproduced in the laboratory to use it as a basis for theanalysis of the effectiveness of this kind of systems according to the heat exchangers, the groutingmaterials and the temperatures of the ground. The components of this prototype of installation werethe following:

• Heat exchangers

Heat exchangers considered in the present research were the commonly used single or doubleU-tube heat exchangers and the helical-shaped pipe, both made of polyethylene and diameterof 32 mm.

• Spacers

In some tests, spacers were placed in single and double U-tube heat exchangers to avoid thecontact between cold and hot pipes. These elements were of polyethylene.

• Boost pump

A boost pump, of 3 W of power, allowed the circulation of the heat carrier fluid through the heatexchangers. The constant volume of flow supplied by this pump was of 6.91 L/h.

• Bucket

The system includes a bucket containing water that simulates the ground at a certain temperature.This bucket has a diameter of 0.45 m and a length of 1 m and is made of polyethylene. The rest ofcomponents of the geothermal drilling: pipes, heat carrier fluid and grouting material are housedinside this bucket.

• Resistant heater

Water contained in the bucket simulating the surrounding ground (considering the groundtemperature as constant during the heat extraction) was set at a temperature thanks to the use ofa resistant heater that keeps the whole water at a constant temperature.

• Heat carrier fluid

The function of the heat carrier fluid is to absorb the heat from the ground by its circulationthrough the geothermal heat exchangers. The heat carrier fluid used in the present study wasa mixture of water-propylene glycol to 30%. It was chosen for being one of the least toxicantifreeze. It does not present any risks for the environment and it can be handled without specialsecurity measures.

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• Grouting material

Several materials are usually used as grout in borehole heat exchangers such as bentonite, sand,cement and detritus coming from the drilling. These materials allow the heat exchange betweenground and pipes and must comply with a series of factors explained in Section 1 [23–27].After testing a series of mixtures, the materials selected as grout were the sand in saturatedconditions, in boreholes with presence of water and a mixture constituted by aluminiumcement-sand, for the case of boreholes without water. Both mixtures have the suitable thermaland mechanical characteristics to be used as grouting material, with significant values of thermalconductivity, 2.83 W/mK in the case of saturated sand and 2.45 W/mK in the mixture ofaluminium cement-sand. Table 2 shows the main characteristics of both materials selectedas grouts in the present work.

• Water

Water contained in the bucket simulates the surrounding ground. The temperature of this fluidwas controlled to study the heat exchange with the rest of components of the installation. Heatingthis water was possible thanks to a resistant heater which allows setting the temperature ata known value. Additionally, temperature of the water was controlled by an external thermometerto verify the correct working of the resistant heater.

• Thermocouples

Inlet and outlet temperatures of the heat carrier fluid were controlled at the end of the heatexchangers by thermocouples connected to a measuring device. These values were essential inthe research to compare the different configurations.

Thermocouples (with an accuracy of ±0.1 ◦C) were constituted by chrome and aluminium alloysand were connected to a digital thermometer to measure simultaneal temperature in differenthorizons or areas. Before its use, these sounding lines were duly calibrated according to theInternational Law ASTM E220 (Test Method for Calibration of Thermocouples by ComparisonTechniques) [28].

• Heat carrier fluid cooling

Once the heat carrier fluid (mixture of water-propylene glycol) has absorbed the heat from theground through the drilling, it gets in a cooling tank where its temperature gradually drops backto start a new cycle of heat exchange with the ground.

Table 2. Main characteristic of the materials chosen to be used as grout.

Parameter Saturated Sand Aluminium Cement-Sand

Composition Silica fine-grain sand completelysaturated by water

Sulpho-aluminate cements ALI CEM (25%),silica fine-grain sand (50%) and water (25%)

Thermal Conductivity (W/mK) 2.83 2.45

Density (Kg/m3·10−3) 2.44 2.10

Hydraulic Conductivity

Very low(When sand is totally saturated,

the hydraulic conductivity is verylow making this material suitable

for it use as grout)

Very low

Compression strength (MPa) - >15 MPa (minimum value recommendedfor cement mixtures)

Retractions It does not experimentretractions of volume

It does not experimentretractions of volume

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Figure 1 shows the schema of the borehole heat exchanger represented in laboratory with thesingle U-tube heat exchanger (Figure 1a) and the helical-shaped pipe (Figure 1b).

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Heat exchangers (vertical or helical) were also inside this bucket and the space between these pipes and the ground (water) was filled by the grouting material.

Finally, a pump placed in the cooled tank of propylene-glycol circulates the heat carrier fluid inside the pipes allowing the absorption of heat on its way through the ground. Thermocouples were placed in the upper part of the pipes to control inlet and outlet temperatures.

Figure 1. Schema of the vertical borehole heat exchanger reproduced in laboratory. (a) Vertical heat exchangers; (b) helical heat exchangers.

2.2. Tested Configurations

Based on the above installation, different configurations were tested modifying three fundamental parameters: heat exchangers, grouting material and ground temperature (water).

2.2.1. Heat Exchangers

Pipes used were the single or double U-tube heat exchangers and the helical-shaped pipe. Different configurations were used in the laboratory tests:

• Single U-tube with spacers: single vertical pipe with spacer placed in the middle of both tubes to avoid any contact between cold and hot pipes.

• Single U-tube without spacers: single vertical pipe without spacers and thus, both tubes (cold and hot) are in contact.

• Double U-tube with spacers: double vertical pipes with spacers to avoid any contact between the four pipes (two cold and two hot).

• Double U-tube without spacers: double vertical pipes without spacers, that is to say, cold and hot tubes are in contact.

• Helical-shaped pipe: helical pipe with a central tube where the heat carrier fluid rises up once has taken the heat from the ground.

Figure 2 shows the influence of the use of spacers relative to the position of the single and double U-tubes heat exchangers.

Figure 1. Schema of the vertical borehole heat exchanger reproduced in laboratory. (a) Vertical heatexchangers; (b) helical heat exchangers.

The bucket contains water that simulates the ground close to the geothermal drilling. This fluid isset at a specific temperature by a heater placed in the upper area of the bucket that recirculates thewhole fluid facilitating a constant temperature at any part of it. A series of thermometers placed indifferent areas of this bucket allowed to verify that the temperature of the water was in effect constant.

Heat exchangers (vertical or helical) were also inside this bucket and the space between thesepipes and the ground (water) was filled by the grouting material.

Finally, a pump placed in the cooled tank of propylene-glycol circulates the heat carrier fluidinside the pipes allowing the absorption of heat on its way through the ground. Thermocouples wereplaced in the upper part of the pipes to control inlet and outlet temperatures.

2.2. Tested Configurations

Based on the above installation, different configurations were tested modifying three fundamentalparameters: heat exchangers, grouting material and ground temperature (water).

2.2.1. Heat Exchangers

Pipes used were the single or double U-tube heat exchangers and the helical-shaped pipe.Different configurations were used in the laboratory tests:

• Single U-tube with spacers: single vertical pipe with spacer placed in the middle of both tubes toavoid any contact between cold and hot pipes.

• Single U-tube without spacers: single vertical pipe without spacers and thus, both tubes (cold andhot) are in contact.

• Double U-tube with spacers: double vertical pipes with spacers to avoid any contact between thefour pipes (two cold and two hot).

• Double U-tube without spacers: double vertical pipes without spacers, that is to say, cold and hottubes are in contact.

• Helical-shaped pipe: helical pipe with a central tube where the heat carrier fluid rises up once hastaken the heat from the ground.

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Figure 2 shows the influence of the use of spacers relative to the position of the single and doubleU-tubes heat exchangers.

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Figure 2. Position of single and double U-tube heat exchangers when using or not spacers. (a) Single U-tubes with spacers; (b) Single U-tubes without spacers; (c) Double U-tubes with spacers; (d) Double U-tubes without spacers.

2.2.2. Grouting Material

As previously cited in Section 2.1, two types of grouts were selected to carry out the present research. These grouts comply with a series of general characteristics making them suitable for this use. Additionally, they have a high thermal conductivity value, that is, they have an excellent capacity to conduce the heat. After preparing mixtures constituted by materials commonly used as grouts, some particular thermal and mechanical tests were carried out [29]. One of these tests, that stands out because of its importance is the measuring “in situ” of the thermal conductivity parameter [30,31]. This property was determined by the thermal properties analyser commercially known as KD2 Pro developed by Decagon Devices [32].

KD2 Pro equipment is constituted by a portable controller and a certain sensor (RK-1) that make possible the measuring of two thermal properties: the thermal resistivity and the thermal conductivity. Its operation is based on the infinite line heat source theory and calculates the thermal conductivity by monitoring the dissipation of heat from the needle probe. Heat is applied to the needle for a set heating time, th and temperature is measured in the monitoring needle during heating and for an additional time equal to th after heating. The temperature during heating is computed from Equation (1). = + + (1)

where, m0 is the ambient temperature during heating; m2 is the rate of background temperature drift; m3 is the slope of a line relating temperature rise to logarithm of temperature.

Equation (2) represents the model during cooling. = + + − (2)

The thermal conductivity is computed from Equation (3). = 4 (3)

The RK-1 probe (3.9 mm in diameter and 6 cm in length), used in the present work, is capable of measuring the thermal conductivity between the range of 0.1 and 6 W/mK and ±10% of accuracy.

The use of this equipment in samples previously prepared (cylinder blocks of 5 cm of diameter and length superior to the sensor RK-1 length) led to select for the present work the following grouts:

• Saturated sand: for those cases of boreholes where water is present, an appropriate solution is to use sand as geothermal grout. When this element is saturated of water (in this case from the borehole) has excellent thermal transmission capacities. It has a thermal conductivity value of 2.83 W/mK.

• Mixture of aluminium cement-sand: this mixture consisting of sand and aluminium cement in proportion 2:1 is suitable for both boreholes with water and without it. Aluminium cement comprised of calcium sulpho-aluminate provides the mixture with a higher thermal

Figure 2. Position of single and double U-tube heat exchangers when using or not spacers. (a) SingleU-tubes with spacers; (b) Single U-tubes without spacers; (c) Double U-tubes with spacers; (d) DoubleU-tubes without spacers.

2.2.2. Grouting Material

As previously cited in Section 2.1, two types of grouts were selected to carry out the presentresearch. These grouts comply with a series of general characteristics making them suitable for thisuse. Additionally, they have a high thermal conductivity value, that is, they have an excellent capacityto conduce the heat. After preparing mixtures constituted by materials commonly used as grouts,some particular thermal and mechanical tests were carried out [29]. One of these tests, that stands outbecause of its importance is the measuring “in situ” of the thermal conductivity parameter [30,31].This property was determined by the thermal properties analyser commercially known as KD2 Prodeveloped by Decagon Devices [32].

KD2 Pro equipment is constituted by a portable controller and a certain sensor (RK-1) that makepossible the measuring of two thermal properties: the thermal resistivity and the thermal conductivity.Its operation is based on the infinite line heat source theory and calculates the thermal conductivity bymonitoring the dissipation of heat from the needle probe. Heat is applied to the needle for a set heatingtime, th and temperature is measured in the monitoring needle during heating and for an additionaltime equal to th after heating. The temperature during heating is computed from Equation (1).

T = m0 + m2t + m3lnt (1)

where, m0 is the ambient temperature during heating; m2 is the rate of background temperature drift;m3 is the slope of a line relating temperature rise to logarithm of temperature.

Equation (2) represents the model during cooling.

T = m1 + m2t + m3lnt

t − th(2)

The thermal conductivity is computed from Equation (3).

k =q

4m3(3)

The RK-1 probe (3.9 mm in diameter and 6 cm in length), used in the present work, is capable ofmeasuring the thermal conductivity between the range of 0.1 and 6 W/mK and ±10% of accuracy.

The use of this equipment in samples previously prepared (cylinder blocks of 5 cm of diameterand length superior to the sensor RK-1 length) led to select for the present work the following grouts:

• Saturated sand: for those cases of boreholes where water is present, an appropriate solution isto use sand as geothermal grout. When this element is saturated of water (in this case from theborehole) has excellent thermal transmission capacities. It has a thermal conductivity value of2.83 W/mK.

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• Mixture of aluminium cement-sand: this mixture consisting of sand and aluminium cementin proportion 2:1 is suitable for both boreholes with water and without it. Aluminium cementcomprised of calcium sulpho-aluminate provides the mixture with a higher thermal conductivityvalue in comparison with the conventional cement. In addition, the mechanical properties of themixture make it appropriate to be used as geothermal grout. It has a thermal conductivity valueof 2.45 W/mK.

2.2.3. Temperatures

Temperatures of the water that simulates the ground were set in several values to cover a range ofpossibilities. It allows analysing the behaviour of the installation with different values of this parameter.Thus, temperature values were set in 30, 40 and 50 ◦C. Temperature of the ground is often underthese values (although it depends on the borehole depth), especially in these installations of very lowenthalpy geothermal energy. However, and given that it is a small installation with a length of onlyone meter, temperature increases between hot and cold sources are required to appreciate significantdifferences among the different assumptions considered.

Cold source was kept constant to the temperature of 6 ◦C. Due to the presence of this source,the heat carrier fluid coming from the drilling, transfers the heat taken from the ground. Once thisfluid has transferred the heat to the cold source, it cools down to start a new cycle.

Table 3 shows each of the configurations tested in laboratory according to the parameterspreviously described. It should be noted that in the case of helical-shaped pipe, the original designof this pipe was used. In this design there is not contact among the different sections of the pipes,therefore, spacers were not used.

Table 3. Different tests experimented in laboratory and temperature and time results.

Test HeatExchanger Spacer Grout Ground T

(◦C)Inlet T

(◦C)Outlet T

(◦C)Increment

(◦C)Time for

Stabilization (s)

T1Single U-Tube X

SaturatedSand

30 18.0 18.9 0.9 1320T2 40 19.7 20.7 1.2 1860T3 50 19.9 21.5 1.6 1980

T4Single U-Tube - Saturated

Sand

30 18.0 18.6 0.6 1119T5 40 19.8 20.7 0.9 1589T6 50 20.0 21.2 1.2 1960

T7Single U-Tube X

A. CementSand

30 18.8 19.5 0.7 2160T8 40 19.1 20.0 0.9 2560T9 50 20.3 21.7 1.4 2721

T10Single U-Tube - A. Cement

Sand

30 18.6 19.1 0.5 2058T11 40 19.3 20.0 0.7 2422T12 50 20.4 21.5 1.1 2712

T13Double U-tube X

SaturatedSand

30 18.5 19.4 0.9 1436T14 40 18.9 20.1 1.2 1887T15 50 19.7 21.2 1.5 2025

T16Double U-tube - Saturated

Sand

30 18.2 18.8 0.6 1421T17 40 19.1 20.1 1.0 1798T18 50 20.1 21.3 1.2 1996

T19Double U-tube X

A. CementSand

30 18.3 19.0 0.7 2153T20 40 18.9 19.8 0.9 2421T21 50 19.8 21.1 1.3 2816

T22Double U-tube - A. Cement

Sand

30 18.5 19.1 0.6 2120T23 40 19.1 19.8 0.7 2315T24 50 20.6 21.7 1.1 2798

T25 Helical-shapedpipe

SaturatedSand

30 25.2 27.0 1.8 2940T26 40 26.0 27.9 1.9 3254T27 50 26.2 28.4 2.2 3621

T28 Helical-shapedpipe

A. CementSand

30 25.4 26.9 1.5 3456T29 40 26.2 27.9 1.7 3987T30 50 26.6 28.7 2.1 4258

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Figure 3 shows several images illustrating the testing process in laboratory. Figure 3a–c representthe three types of heat exchangers used in the present research and the rest of Figure 3d–f showdifferent views of the installation reproduced in laboratory.

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Figure 3 shows several images illustrating the testing process in laboratory. Figure 3a–c represent the three types of heat exchangers used in the present research and the rest of Figure 3d–f show different views of the installation reproduced in laboratory.

Figure 3. Images of the different configurations tested in laboratory. (a) Single U-tube heat exchanger with spacer; (b) Double U-tube heat exchanger without spacers; (c) Helical-shaped pipe; (d) Single U-tube heat exchanger without spacers and grout of saturated sand; (e) Single U-tube heat exchanger with spacers and grout of the mixture aluminium cement-sand; (f) Double U-tube heat exchanger with spacers and grout of saturated sand.

3. Results and Discussion

During the test of each of the configurations presented in Table 3, temperatures of the heat carrier fluid were continuously controlled by thermocouples placed at the top of the pipes (in both cold and hot tubes). Once these temperatures were stable (time for stabilization was also measured), they were registered. In this way, the increments of temperatures reached by this fluid (when temperatures were stable) constitute an indication of efficiency of the system tested in each particular case. This fact allows for stablishing a comparison among the set of systems represented in laboratory.

Table 3 includes the tests carried out in laboratory and the results of the thermocouples measurements for each of the configurations analysed and the increment reached in each of them. Times for stabilization of inlet and outlet temperatures were also measured and are equally presented in Table 3. Thermocouples were previously calibrated before use and each of the tests were repeated three times to avoid any kind of measurement error. Temperature increments presented in Table 3 are the average of the three tests made for each assumption with identical conditions.

Data recorded by the thermocouples allows showing in Figure 4 a plot of temperatures before stabilization for test T3 (see Table 3). The trend of temperatures for the rest of tests was very similar, so only this test will be shown as an example.

Figure 3. Images of the different configurations tested in laboratory. (a) Single U-tube heat exchangerwith spacer; (b) Double U-tube heat exchanger without spacers; (c) Helical-shaped pipe; (d) SingleU-tube heat exchanger without spacers and grout of saturated sand; (e) Single U-tube heat exchangerwith spacers and grout of the mixture aluminium cement-sand; (f) Double U-tube heat exchanger withspacers and grout of saturated sand.

3. Results and Discussion

During the test of each of the configurations presented in Table 3, temperatures of the heat carrierfluid were continuously controlled by thermocouples placed at the top of the pipes (in both cold andhot tubes). Once these temperatures were stable (time for stabilization was also measured), they wereregistered. In this way, the increments of temperatures reached by this fluid (when temperatures werestable) constitute an indication of efficiency of the system tested in each particular case. This factallows for stablishing a comparison among the set of systems represented in laboratory.

Table 3 includes the tests carried out in laboratory and the results of the thermocouplesmeasurements for each of the configurations analysed and the increment reached in each of them.Times for stabilization of inlet and outlet temperatures were also measured and are equally presentedin Table 3. Thermocouples were previously calibrated before use and each of the tests were repeatedthree times to avoid any kind of measurement error. Temperature increments presented in Table 3 arethe average of the three tests made for each assumption with identical conditions.

Data recorded by the thermocouples allows showing in Figure 4 a plot of temperatures beforestabilization for test T3 (see Table 3). The trend of temperatures for the rest of tests was very similar,so only this test will be shown as an example.

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Figure 4. Plot of temperatures before stabilization for test T3.

3.1. Comparison of Results

A series of considerations about the parameters studied can be derived from the analysis of Table 4.

Table 4. Comparison helical-shaped pipe and single U-tube heat exchanger.

Parameter Helical-Shaped Pipe Single U-Tube Drilling length (m) 1.00 1.00

Total pipe length (m) 6.91 6.91 Flow rate (L/h) 1.92 × 10−6 1.92 × 10−6

Drilling diameter (m) 0.35 0.11 ∆T between cold and hot pipes (°C) 1.8 * 0.9 *

Time to stabilize (s) 1320 3456 * Values corresponding to the ground temperature of 30 °C and the grout of saturated sand.

3.1.1. Ground Temperatures

In all tests, when the temperature of the ground increases, the increment of temperature reached by the heat carrier fluid also grows, as expected given the higher difference between the cold and hot source (ground). As a result, the highest increments were obtained for the ground temperature of 50 °C.

3.1.2. Heat Exchangers

Three types of heat exchangers (single U-tube, double U-tube and helical-shaped pipe) were used. In the cases of vertical pipes, spacers were used in some of the tests.

U-Tube Heat Exchangers

Single and double U-tube heat exchangers (with and without spacers), were used in some of the laboratory tests.

In the first place, the influence of using spacers in U-tube heat exchangers is analysed. As shown in Table 4, the highest temperature increments (in the case of U-tube heat exchangers) correspond to those tests in which spacers were used. In either case, single or double U-tube heat exchangers, the use of spacers allows to increase the temperature increment by an average of 0.3 °C (a fairly high value considering the limited length of the installation). When both tubes (cold and hot) are in contact, the thermal exchange is affected, and hence, the efficiency of the geothermal installation is also affected.

Another important issue to tackle in this research is the difference between using single or double U-tube heat exchangers. Reanalysing Table 4, it is possible to observe that, for the same

Figure 4. Plot of temperatures before stabilization for test T3.

3.1. Comparison of Results

A series of considerations about the parameters studied can be derived from the analysis ofTable 4.

Table 4. Comparison helical-shaped pipe and single U-tube heat exchanger.

Parameter Helical-Shaped Pipe Single U-Tube

Drilling length (m) 1.00 1.00Total pipe length (m) 6.91 6.91

Flow rate (L/h) 1.92 × 10−6 1.92 × 10−6

Drilling diameter (m) 0.35 0.11∆T between cold and hot pipes (◦C) 1.8 * 0.9 *

Time to stabilize (s) 1320 3456

* Values corresponding to the ground temperature of 30 ◦C and the grout of saturated sand.

3.1.1. Ground Temperatures

In all tests, when the temperature of the ground increases, the increment of temperature reachedby the heat carrier fluid also grows, as expected given the higher difference between the cold andhot source (ground). As a result, the highest increments were obtained for the ground temperatureof 50 ◦C.

3.1.2. Heat Exchangers

Three types of heat exchangers (single U-tube, double U-tube and helical-shaped pipe) were used.In the cases of vertical pipes, spacers were used in some of the tests.

U-Tube Heat Exchangers

Single and double U-tube heat exchangers (with and without spacers), were used in some of thelaboratory tests.

In the first place, the influence of using spacers in U-tube heat exchangers is analysed. As shownin Table 4, the highest temperature increments (in the case of U-tube heat exchangers) correspondto those tests in which spacers were used. In either case, single or double U-tube heat exchangers,the use of spacers allows to increase the temperature increment by an average of 0.3 ◦C (a fairly highvalue considering the limited length of the installation). When both tubes (cold and hot) are in contact,the thermal exchange is affected, and hence, the efficiency of the geothermal installation is also affected.

Another important issue to tackle in this research is the difference between using single or doubleU-tube heat exchangers. Reanalysing Table 4, it is possible to observe that, for the same conditions

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of grout and ground temperature, the increments of temperature (in the heat carrier fluid) obtainedwith the use of single U-tube heat exchangers were, in all instances, identical or very similar to theones reached with double U-tube heat exchangers. This information allows concluding that the use ofdouble U-tube heat exchangers does not produce any improvement in terms of efficiency compared tothe use of single U-tube heat exchangers.

Helical-Shaped Pipes

The other type of heat exchanger analysed in the present research is the helical-shaped pipe.All tests made with this pipe showed that the heat exchange between the ground and the heat carrierfluid is more efficient regarding single and double U-tube heat exchangers and considering the samedrilling length. Thus, the increments of temperature presented in Table 3, are in all cases higher (morethan double in some cases) than the ones reached with vertical pipes (either single or double U-tubes).

3.1.3. Grouting Material

The last variable to analyse is the material used as grout filling the space between ground andheat exchangers. All configurations represented in laboratory were tested with two types of grouts.With the first of them, saturated sand, the increments of temperature registered were higher thanthe ones obtained with the mixture of aluminium cement-sand and keeping the same conditions.It was highly expected because, as mentioned above, the thermal conductivity of the saturated sand ishigher than the cement-based grout thermal conductivity. In all cases, differences of around 0.2 ◦Cwere registered between both grouts. A significant value for the high similarity between thermalconductivities (2.83 W/mK saturated sand and 2.45 W/mK the mixture of aluminium cement-sand).

3.1.4. Time for Stabilization

Times for stabilization of inlet and outlet temperatures were measured to complete the comparisonamong the different systems.

In the first place, the highest times presented in Table 4 correspond in each system to the highestdifference of temperature between hot and cold source (for the ground temperature of 50 ◦C).

With respect to the grouting material, saturated sand always involves lower times than the groutof aluminium cement-sand. This happens because heat is transmitted in an easier and faster waythroughout saturated sand given its higher thermal conductivity.

Finally, the most important comparison of this parameter is related to the heat exchanger used.The lowest times for stabilization belong to single U-tube heat exchangers. The shorter length of thissystem facilitates the stabilization of temperatures between cold and hot sources. On the contrary,the helical-shape pipes present the highest times for stabilization given the total length of these pipes.

Although this parameter is not decisive when selecting the most suitable heat exchanger, it isconvenient to consider the different behaviour of each system with respect to the stabilization time.

The most relevant results agree with the conclusions of other similar researches cited in Section 1.The efficiency of a vertical closed-loop system is highly conditioned by the thermal conductivity ofground and grouting material.

In addition, helical heat exchanger constitutes the best option to reach the highest thermalexchange in the installation. In these previous works, these systems are supposed to be the mosteconomical solution. This aspect will be address in Section 3.3.

3.2. Proposed Systems

Following the above considerations, the most appropriate configurations to be used in verticalclose-loop systems are listed below:

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• Helical-shaped pipe using saturated sand or mixture of aluminium cement-sand as grout basedon the conditions of the borehole. The presence of water in the borehole limits the use of saturatedsand as geothermal grout; however, it will be selected wherever possible.

• Single U-tube heat exchanger with spacers to avoid the contact between inlet and outlet pipes.The suggested grouts are also the saturated sand (when conditions allow) and the mixture ofaluminium cement-sand.

Although the drilling length tested for each of these configurations was identical, the designof these two heat exchangers is not the same. Therefore, it is appropriate to make a more detailcomparison between both pipes regarding the system tested in the laboratory. Table 4 includes thecomparison between both heat exchangers concerning a series of parameters.

The most remarkable parameters of Table 4 are the total length of pipes and the drilling diameter.While the length of the drilling of the laboratory tests was the same for both heat exchangers,

the total length of these pipes differs substantially between them. The helical-shaped pipe, given itshelical design, requires a longer pipe (more than double) than the one used by the single U-tube heatexchanger. It causes an increase in the total cost of the borehole heat exchanger for the helical-shapedpipes. However, and as was verified before, the system efficiency (on equal terms) is higher for thesehelical heat exchangers. Thus, to achieve the same efficiency, single U-tube heat exchangers requirea longer drilling length.

Single U-tube heat exchanger would need the total pipe length of the helical-shaped pipe (8.06 m)to reach the same increments of temperature obtained by this heat exchanger in the laboratory tests.Considering the design of the single U-tube heat exchanger constituted by an inlet and an outlet pipe,the drilling length required would be of at least 4.12 m.

The other parameter that differentiates these heat exchangers is the drilling diameter they require.To achieve the same efficiency, the diameter needed to install the helical-shaped pipe would be of atleast 0.35 m in contrast to the diameter of 0.11 m that would be enough in the case of single U-tubeheat exchanger.

Thus, under equal conditions in the borehole heat exchanger, single U-tube heat exchangersrequire more drilling length but fewer diameters than helical-shaped pipes need (double or even morethan vertical heat exchangers).

3.3. Practical Example

As an example, the drilling length required to cover some specific energy needs was calculatedby the software “Earth Energy Designer” (EED). This software, developed by “Blocon Software” [26],allows determining the most important parameters of a vertical closed-loop system: the number ofholes and the drilling depth. Given that the software only refers to U-tube heat exchangers, simulationwas carried out considering the use of a single U-tube heat exchanger with spacers. Based on thiscalculation, the drilling length that the use of a helical-shaped pipe would involve (for the sameassumption) was estimated according to laboratory results. The economic difference between bothconfigurations was also calculated.

The process of calculation of EED is based on a series of initial data (provided by the user)of the ground where the installation is going to be placed and the heat exchanger and heat carrierfluid selected.

Table 5 shows the input data of the supposed vertical geothermal system.Using the parameters presented in Table 5, software EED calculated the drilling length required

to cover the energy demand in question. Over the calculation, this software offers several optionsregarding the drilling length and number of holes. The first of them is always the most suitable option(with the lowest length and number of holes).

Additionally, Table 6 provides an estimation of the cost corresponding to the process of drilling bythe rotary-percussive technique given that the system is considered to be placed in a granitic ground.

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Based on the information supplied by a Spanish Company specialized in drillings “Sondeos Seymar”,the cost per meter of drilling for a borehole of 0.11 m of diameter placed in a granitic ground is about50 €/m. Table 6 shows output data from the calculation of the borehole heat exchanger by EED.

Table 5. Input data used in the calculation with the software “Earth Energy Designer”.

Ground (Granitic Origin)

Thermal conductivity W/(m·K) 2.5Volumetric heat capacity MJ/(m3·K) 2.16Average annual temperature of the surface ◦C 8Heat flow W/m2 0.06

Heat Carrier Fluid

Thermal conductivity W/(m·K) 0.47Mass heat capacity J/(Kg·K) 3930Density Kg/m3 1033Viscosity Kg/(m·s) 0.0079Freezing point ◦C −10Flow rate per hole L/s 2

Grouting Material

Thermal conductivity W/(m·K) 2.83

Heat Exchanger

Configuration Vertical Simple-UPipe diameter (m) 0.032

Basic Demand

Annual demand of SHW MWh 5Heat annual demand MWh 16.2Cooling annual demand MWh 0Seasonal operation coefficient (ACS) 3Seasonal COP (heating) 3Seasonal COP (cooling) 3

Table 6. Calculation of the geothermal drilling by the software “Earth Energy Designer” using singleU-tube heat exchangers. Value provided by the Spanish company “Sondeos Seymar”.

Single U-Tube Heat Exchangers

Number of holes 1Drilling depth (m) 110

Total drilling length (m) 110Cost per meter of drilling (€/m) 50

Total drilling cost (€) 5.500

From values presented in Table 6 (corresponding to a single U-tube heat exchanger), it waspossible to estimate the parameters for a helical-shaped pipe considering the same thermal efficiencyand identical input data. Using the relation deduced in this research (4.12 m of drilling with singleU-tube heat exchanger per each meter of drilling with the helical-shaped pipe), output data werecalculated for the helical heat exchanger. These output data are included in Table 7. In this case, thedrilling unit cost for this pipe is considered as higher given the higher drilling diameter (0.35 m) theseexchangers need. This cost, provided by “Sondeos Seymar” for a granitic ground is of around 90 €/mof drilling.

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Table 7. Calculation of the geothermal drilling using helical-shaped pipes. Value provided by theSpanish company “Sondeos Seymar”.

Helical-Shaped Pipes

Number of holes 1Drilling depth (m) 27

Total drilling length (m) 27Cost per meter of drilling (€/m) 90

Total drilling cost (€) 2.430

4. Conclusions

The present work analyses and tests the most common configurations used in vertical closed-loopsystems of very low enthalpy. The behaviour of these installations is studied based on the heatexchangers, grouting material and temperatures of the ground.

It is important to mention that this work made some simplifications:

• The subsoil temperature was kept constant underground;• The long-term depletion of underground was not considered.

As a result of the laboratory tests described throughout this work, some conclusions are deduced:

• Helical-shaped pipes are the best solution to provide the highest efficiency in the thermal exchangebetween ground and heat carrier fluid. For the same drilling length, these heat exchangers improvethe performance of the borehole heat exchanger regarding single/double U-tube heat exchangers(with or without spacers).

• Helical-shaped pipes allow reducing the total drilling length required to cover some particularenergy needs. Under the same thermal efficiency conditions, vertical heat exchangers require adrilling length four times greater than that required by the helical pipes. The use of these helicalheat exchangers involves important economic savings in spite of the higher drilling diameterthey need.

• Double U-tube heat exchangers do not provide significant improvements in the process of thermalexchange in relation to single U-tube heat exchangers. Laboratory tests results reveal that the useof single U-tube heat exchangers supply the same efficiency than the double ones. The use ofthese double vertical pipes would only be an interesting solution if one of the U-tubes failed orbecame blocked so the other U-tube could go on working.

• The use of spacers in vertical U-tube heat exchangers offer better results than in those cases whereinlet and outlet pipes are in contact. Laboratory tests have shown improvements of around 30%when using tubes with these separating elements.

• As expected, the higher thermal conductivity of the grouting material, the greater efficiency ofthe process of heat gaining by the heat carrier fluid. This fact was verified by testing two groutswith different thermal conductivity values. Thus, the most thermally conductive grout (saturatedsand) provided the best result of thermal exchange.

In future researches, a calibration of the results presented in this manuscript will be carried outthrough a thermal resistance capacity model “TRCM” with the aim of making a comparison with thetheoretical value. Finally, studying the behaviour of the systems considered by modifying the flowinserted to the pipes would be interesting to enrich the present study.

Acknowledgments: Authors would like to thank the Department of Cartographic and Land Engineering of theHigher Polytechnic School of Avila, University of Salamanca, for allowing us to use their facilities and theircollaboration during the experimental phase of this research. Authors also want to thank the Ministry of Education,Culture and Sport for providing a FPU Grant (Training of University Teachers Grant) to the corresponding authorof this paper what has made possible the realization of the present work.

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Author Contributions: All authors conceived, designed and performed the experimental campaign.Cristina Sáez Blázquez, Arturo Farfán Martín and Ignacio Martín Nieto implemented the methodology andanalyzed the results. Pedro Carrrasco García, Luis Santiago Sánchez Pérez and Diego González-Aguilera providedtechnical and theoretical support. Cristina Sáez Blázquez wrote the manuscript and all authors read and approvedthe final version.

Conflicts of Interest: The authors declare no conflicts of interest.

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